1. Field of the Invention
The present invention relates to a copying apparatus capable of detecting the density of an original document and controlling the image forming conditions according to the result of said detection.
2. Description of the Prior Art
The density control in a conventional copier has generally been achieved by a manual continuous control lever for example movable within a range from "F1" to "F9", or by a stepwise selector means suitably positioned for example at "dark", "medium" or "light".
In such manual density control, if an original with a relatively dark background, for example newspaper, diazo copy or original printed on colored paper, is copied with the density control adjusted at the standard density, there will be obtained a copy with so-called background fog, in which the background color of the original is reproduced in the same color as that of the reproduced image so that the entire copy looks smeared. On the other hand, in the case of an original with an extremely low image density, for example an original with a hard pencil, the image may not be faithfully reproduced on the copy.
In order to prevent such phenomena, the operator is required to adjust the density control lever to a position "F8" to "F9" or the selector to a position "light" in the former case, or to adjust the density control lever to a position "F3" or "F4" or the selector to a position "dark" in the latter case, in anticipation of the result.
FIG. 1 shows the conventional manual density control device, composed of a density control variable resistor 1 and a copy density scale 2, wherein "1" indicates a dark image density used in case of obtaining a higher image density from a low density original, such as one written with a hard pencil. A position "9" indicates a light image density, utilized for copying an original with colored background, for example newspaper or diazo copy, without reproducing said background color. A position "5" corresponds to a normal image density. Thus the operator has been required to suitably adjust the density control variable resistor 1 within the range from "1" to "9", according to the original document to be copied.
In such operation, however, an optimum copy can only be obtained when the operator has become used to the original after making plural copies from the same original, so that unnecessary waste copies have been unavoidable because of inadequate density control for an original newly encountered.
In order to avoid such inconvenience there is already developed a copying apparatus with automatic density controlling function. Such copier is designed to adjust the image density automatically by reading the density of the original document and accordingly adjusting the exposure or the image development.
Such density control would become ideal if the background color of the original can be detected. In practice, however, detection is generally made on the average density of the original since the measurement of background color of various originals is extremely difficult. Consequently such automatic density control is still unable to cover all the originals and results in waste copies. Furthermore the density level determined by the copier may be different from what is desired by the user. Because of such factors manual density control may still be required in the copier with the automatic density control function.